Structure of Crumbs tail in complex with the PALS1 PDZ-SH3-GK tandem reveals a highly specific assembly mechanism for the apical Crumbs complex

Entosis implicates a new role for P53 in microcephaly pathogenesis, beyond apoptosis

Entosis, a form of cell cannibalism, is a newly discovered pathogenic mechanism leading to the development of small brains, termed microcephaly, in which P53 activation was found to play a major role. Microcephaly with entosis, found in Pals1 mutant mice, displays P53 activation that promotes entosis and apoptotic cell death. This previously unappreciated pathogenic mechanism represents a novel cellular dynamic in dividing cortical progenitors which is responsible for cell loss. To date, various recent models of microcephaly have bolstered the importance of P53 activation in cell death leading to microcephaly. P53 activation caused by mitotic delay or DNA damage manifests apoptotic cell death which can be suppressed by P53 removal in these animal models. Such genetic studies attest P53 activation as quality control meant to eliminate genomically unfit cells with minimal involvement in the actual function of microcephaly associated genes. In this review, we summarize the known role of P53 activation in a variety of microcephaly models and introduce a novel mechanism wherein entotic cell cannibalism in neural progenitors is triggered by P53 activation.

Genetic evidence for splicing-dependent structural and functional plasticity in CASK protein

BACKGROUND

Pontocerebellar hypoplasia (PCH) may present with supratentorial phenotypes and is often accompanied by microcephaly. Damaging mutations in the X-linked gene CASK produce self-limiting microcephaly with PCH in females but are often lethal in males. CASK deficiency leads to early degeneration of cerebellar granule cells but its role in other regions of the brain remains uncertain.

METHOD

We generated a conditional Cask knockout mice and deleted Cask ubiquitously after birth at different times. We examined the clinical features in several subjects with damaging mutations clustered in the central part of the CASK protein. We have performed phylogenetic analysis and RT-PCR to assess the splicing pattern within the same protein region and performed in silico structural analysis to examine the effect of splicing on the CASK's structure.

RESULT

We demonstrate that deletion of murine Cask after adulthood does not affect survival but leads to cerebellar degeneration and ataxia over time. Intriguingly, damaging hemizygous CASK mutations in boys who display microcephaly and cerebral dysfunction but without PCH are known. These mutations are present in two vertebrate-specific CASK exons. These exons are subject to alternative splicing both in forebrain and hindbrain. Inclusion of these exons differentially affects the molecular structure and hence possibly the function/s of the CASK C-terminus.

CONCLUSION

Loss of CASK function disproportionately affects the cerebellum. Clinical data, however, suggest that CASK may have additional vertebrate-specific function/s that play a role in the mammalian forebrain. Thus, CASK has an ancient function shared between invertebrates and vertebrates as well as novel vertebrate-specific function/s.

Intestinal mRNA expression analysis of polarity-related genes identified the discriminatory ability of CRB3 as a diagnostic marker for celiac disease

BACKGROUND

Celiac disease (CD) is a chronic autoimmune disorder characterized by an abnormal immune response to gluten, a protein found in wheat, barley, and rye. It is well established that the integrity of epithelial tight junctions (TJs) and adherens junctions (AJs) plays a crucial role in the pathogenesis of CD. These junctional complexes contribute to the apical-basal polarity of the intestinal epithelial cells, which is crucial for their proper functioning.

METHODS

Sixty CD subjects, and 50 controls were enrolled in the current study. Mucosal samples were obtained from the distal duodenum, total RNA was extracted and complementary DNA was synthesized. The relative expression levels of the desired genes were evaluated by quantitative real-time polymerase chain reaction based on ΔΔCt method. The gene-gene interaction network was also constructed using GeneMANIA.

RESULTS

CRB3 (p = .0005), LKB1 (p < .0001), and SCRIB (p = .0005) had lower expression in CD patients compared to controls, while PRKCZ expression did not differ between groups (p > .05). CRB3 represented a significant diagnostic value for differentiating CD patients from the control group (p = .02).

CONCLUSION

The aim of the current study was to evaluate the changes in the mRNA expression levels of SCRIB, PRKCZ, LKB1, and CRB3 genes in the small intestinal biopsy samples of CD patients in comparison to the healthy control subjects. Our data uncover the importance of polarity-related genes (especially CRB3) in CD pahtomechanism, that may facilitate the planning of the future studies looking for finding innovative diagnostic and therapeutic strategies for CD.

A de novo missense mutation in MPP2 confers an increased risk of Vogt-Koyanagi-Harada disease as shown by trio-based whole-exome sequencing

Vogt-Koyanagi-Harada (VKH) disease is a leading cause of blindness in young and middle-aged people. However, the etiology of VKH disease remains unclear. Here, we performed the first trio-based whole-exome sequencing study, which enrolled 25 VKH patients and 50 controls, followed by a study of 2081 VKH patients from a Han Chinese population to uncover detrimental mutations. A total of 15 de novo mutations in VKH patients were identified, with one of the most important being the membrane palmitoylated protein 2 (MPP2) p.K315N (MPP2-N315) mutation. The MPP2-N315 mutation was highly deleterious according to bioinformatic predictions. Additionally, this mutation appears rare, being absent from the 1000 Genome Project and Genome Aggregation Database, and it is highly conserved in 10 species, including humans and mice. Subsequent studies showed that pathological phenotypes and retinal vascular leakage were aggravated in MPP2-N315 mutation knock-in or MPP2-N315 adeno-associated virus-treated mice with experimental autoimmune uveitis (EAU). In vitro, we used clustered regularly interspaced short palindromic repeats (CRISPR‒Cas9) gene editing technology to delete intrinsic MPP2 before overexpressing wild-type MPP2 or MPP2-N315. Levels of cytokines, such as IL-1β, IL-17E, and vascular endothelial growth factor A, were increased, and barrier function was destroyed in the MPP2-N315 mutant ARPE19 cells. Mechanistically, the MPP2-N315 mutation had a stronger ability to directly bind to ANXA2 than MPP2-K315, as shown by LC‒MS/MS and Co-IP, and resulted in activation of the ERK3/IL-17E pathway. Overall, our results demonstrated that the MPP2-K315N mutation may increase susceptibility to VKH disease.

A Size Filter Regulates Apical Protein Sorting

Despite decades of research, apical sorting of epithelial membrane proteins remains incompletely understood. We noted that apical cytoplasmic domains are smaller than those of basolateral proteins; however, the reason for this discrepancy is unknown. We investigated whether a size barrier at the trans-Golgi network (TGN) might hinder apical sorting of proteins with large cytoplasmic tails. We focused on Crb3 and Ace2 as representative apical proteins with short cytoplasmic tails. By incorporating a streptavidin-binding peptide, these proteins can be trapped in the endoplasmic reticulum (ER) until addition of biotin, which triggers synchronous release to the Golgi and subsequent transport to the apical cortex. Strikingly, departure from the Golgi could be significantly delayed simply by increasing cytoplasmic bulk. Moreover, large and small Crb3 segregated into spatially distinct Golgi regions as detected by super resolution imaging. Biologically, Crb3 forms a complex through its cytoplasmic tail with the Pals1 protein, which could also delay departure, but although associated at the ER and Golgi, we found that Pals1 disassociates prior to Crb3 departure. Notably, a non-dissociable mutant Pals1 hampers the exit of Crb3. We conclude that an unexpected mechanism involving a size filter at the TGN facilitates apical sorting of proteins with small cytoplasmic domains and that timely release of Pals1, to reduce cytoplasmic domain size, is essential for the normal kinetics of Crb3 sorting.

SARS-CoV-2 E protein: Pathogenesis and potential therapeutic development

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a devastating global pandemic, which has seriously affected human health worldwide. The discovery of therapeutic agents is extremely urgent, and the viral structural proteins are particularly important as potential drug targets. SARS-CoV-2 envelope (E) protein is one of the main structural proteins of the virus, which is involved in multiple processes of the virus life cycle and is directly related to pathogenesis process. In this review, we present the amino acid sequence of the E protein and compare it with other two human coronaviruses. We then explored the role of E protein in the viral life cycle and discussed the pathogenic mechanisms that E protein may be involved in. Next, we summarize the potential drugs against E protein discovered in the current studies. Finally, we described the possible effects of E protein mutation on virus and host. This established a knowledge system of E protein to date, aiming to provide theoretical insights for mitigating the current COVID-19 pandemic and potential future coronavirus outbreaks.

Regulation of Liprin-α phase separation by CASK is disrupted by a mutation in its CaM kinase domain

Mutations in the human CASK gene cause a neurodevelopmental disorder; we show that CASK regulates condensate formation of Liprin-alpha 2 and that patient mutations in the CaM kinase domain interfere with Liprin binding and regulation of condensate formation.

CASK is a unique membrane-associated guanylate kinase (MAGUK) because of its Ca²⁺/calmodulin-dependent kinase (CaMK) domain. We describe four male patients with a severe neurodevelopmental disorder with microcephaly carrying missense variants affecting the CaMK domain. One boy who carried the p.E115K variant and died at an early age showed pontocerebellar hypoplasia (PCH) in addition to microcephaly, thus exhibiting the classical MICPCH phenotype observed in individuals with CASK loss-of-function variants. All four variants selectively weaken the interaction of CASK with Liprin-α2, a component of the presynaptic active zone. Liprin-α proteins form spherical phase-separated condensates, which we observe here in Liprin-α2 overexpressing HEK293T cells. Large Liprin-α2 clusters were also observed in transfected primary-cultured neurons. Cluster formation of Liprin-α2 is reversed in the presence of CASK; this is associated with altered phosphorylation of Liprin-α2. The p.E115K variant fails to interfere with condensate formation. As the individual carrying this variant had the severe MICPCH disorder, we suggest that regulation of Liprin-α2–mediated phase condensate formation is a new functional feature of CASK which must be maintained to prevent PCH.

Case Report: Identification of a novel CASK missense variant in a Chinese family with MICPCH

Mental retardation and microcephaly with pontine and cerebellar hypoplasia (MICPCH) is a rare genetic disorder that results in varying levels of pontocerebellar hypoplasia, microcephaly, and severe intellectual disabilities. Prior genetic analyses have identified the CASK gene as a driver of MICPCH. Herein, we analyzed a Chinese family with MICPCH. The index patient was an 8-year-old male. He and his 3-year-old brother suffered from microcephaly, pontocerebellar hypoplasia, serious mental retardation, ataxia, gait disorder, and inability to speak. Through a combination of whole-exome sequencing and subsequent Sanger sequencing, a novel X-linked missense mutation, c.1882G&gt;C (p.D628H) in the CASK gene, was identified in two siblings, as well as their mother and grandmother, who exhibited mild mental retardation. Other family members with negative genetic testing were normal. In silico analyses indicated that this missense mutation was predicted to reduce CASK protein stability, disrupt the SRC homology 3 (SH3) domain, and abolish its function. In summary, we identified a novel missense variate in CASK associated with MICPCH. Our work facilitates the diagnosis of the disease in this family and broadens the gene variant spectrum of the CASK in MICPCH patients.

Identification and characterization of Crumbs polarity complex proteins in Caenorhabditis elegans

Crumbs proteins are evolutionarily conserved transmembrane proteins with essential roles in promoting the formation of the apical domain in epithelial cells. The short intracellular tail of Crumbs proteins are known to interact with several proteins, including the scaffolding protein PALS1 (protein associated with LIN7, Stardust in Drosophila). PALS1 in turn binds to a second scaffolding protein PATJ (PALS1-associated tight junction protein) to form the core Crumbs/PALS1/PATJ complex. While essential roles in epithelial organization have been shown for Crumbs proteins in Drosophila and mammalian systems, the three Caenorhabditis elegans crumbs genes are dispensable for epithelial polarization and development. Here, we investigated the presence and function of PALS1 and PATJ orthologs in C. elegans. We identified MAGU-2 as the C. elegans ortholog of PALS1 and show that MAGU-2 interacts with all three Crumbs proteins and localizes to the apical membrane domain of intestinal epithelial cells in a Crumbs-dependent fashion. Similar to crumbs mutants, magu-2 deletion showed no epithelial polarity defects. We also identified MPZ-1 as a candidate ortholog of PATJ based on the physical interaction with MAGU-2 and sequence similarity with PATJ proteins. However, MPZ-1 is not broadly expressed in epithelial tissues and, therefore, not likely a core component of the C. elegans Crumbs complex. Finally, we show overexpression of the Crumbs proteins EAT-20 or CRB-3 can lead to apical membrane expansion in the intestine. Our results shed light on the composition of the C. elegans Crumbs complex and indicate that the role of Crumbs proteins in promoting apical domain formation is conserved.

The Non-Linear Path from Gene Dysfunction to Genetic Disease: Lessons from the MICPCH Mouse Model

Most human disease manifests as a result of tissue pathology, due to an underlying disease process (pathogenesis), rather than the acute loss of specific molecular function(s). Successful therapeutic strategies thus may either target the correction of a specific molecular function or halt the disease process. For the vast majority of brain diseases, clear etiologic and pathogenic mechanisms are still elusive, impeding the discovery or design of effective disease-modifying drugs. The development of valid animal models and their proper characterization is thus critical for uncovering the molecular basis of the underlying pathobiological processes of brain disorders. MICPCH (microcephaly and pontocerebellar hypoplasia) is a monogenic condition that results from variants of an X-linked gene, CASK (calcium/calmodulin-dependent serine protein kinase). CASK variants are associated with a wide range of clinical presentations, from lethality and epileptic encephalopathies to intellectual disabilities, microcephaly, and autistic traits. We have examined CASK loss-of-function mutations in model organisms to simultaneously understand the pathogenesis of MICPCH and the molecular function/s of CASK. Our studies point to a highly complex relationship between the potential molecular function/s of CASK and the phenotypes observed in model organisms and humans. Here we discuss the implications of our observations from the pathogenesis of MICPCH as a cautionary narrative against oversimplifying molecular interpretations of data obtained from genetically modified animal models of human diseases.

Structural biology of SARS-CoV-2: open the door for novel therapies

Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) is the causative agent of the pandemic disease COVID-19, which is so far without efficacious treatment. The discovery of therapy reagents for treating COVID-19 are urgently needed, and the structures of the potential drug-target proteins in the viral life cycle are particularly important. SARS-CoV-2, a member of the Orthocoronavirinae subfamily containing the largest RNA genome, encodes 29 proteins including nonstructural, structural and accessory proteins which are involved in viral adsorption, entry and uncoating, nucleic acid replication and transcription, assembly and release, etc. These proteins individually act as a partner of the replication machinery or involved in forming the complexes with host cellular factors to participate in the essential physiological activities. This review summarizes the representative structures and typically potential therapy agents that target SARS-CoV-2 or some critical proteins for viral pathogenesis, providing insights into the mechanisms underlying viral infection, prevention of infection, and treatment. Indeed, these studies open the door for COVID therapies, leading to ways to prevent and treat COVID-19, especially, treatment of the disease caused by the viral variants are imperative.

Targeting PDZ domains as potential treatment for viral infections, neurodegeneration and cancer

The interaction between proteins is a fundamental event for cellular life that is generally mediated by specialized protein domains or modules. PDZ domains are the largest class of protein-protein interaction modules, involved in several cellular pathways such as signal transduction, cell-cell junctions, cell polarity and adhesion, and protein trafficking. Because of that, dysregulation of PDZ domain function often causes the onset of pathologies, thus making this family of domains an interesting pharmaceutical target. In this review article we provide an overview of the structural and functional features of PDZ domains and their involvement in the cellular and molecular pathways at the basis of different human pathologies. We also discuss some of the strategies that have been developed with the final goal to hijack or inhibit the interaction of PDZ domains with their ligands. Because of the generally low binding selectivity of PDZ domain and the scarce efficiency of small molecules in inhibiting PDZ binding, this task resulted particularly difficult to pursue and still demands increasing experimental efforts in order to become completely feasible and successful in vivo.

De novo variants in MPP5 cause global developmental delay and behavioral changes

Membrane Protein Palmitoylated 5 (MPP5) is a highly conserved apical complex protein essential for cell polarity, fate and survival. Defects in cell polarity are associated with neurologic disorders including autism and microcephaly. MPP5 is essential for neurogenesis in animal models, but human variants leading to neurologic impairment have not been described. We identified three patients with heterozygous MPP5 de novo variants (DNV) and global developmental delay (GDD) and compared their phenotypes and magnetic resonance imaging (MRI) to ascertain how MPP5 DNV leads to GDD. All three patients with MPP5 DNV experienced GDD with language delay/regression and behavioral changes. MRI ranged from normal to decreased gyral folding and microcephaly. The effects of MPP5 depletion on the developing brain were assessed by creating a heterozygous conditional knock out (het CKO) murine model with central nervous system (CNS)-specific Nestin-Cre drivers. In the het CKO model, Mpp5 depletion led to microcephaly, decreased cerebellar volume and cortical thickness. Het CKO mice had decreased ependymal cells and Mpp5 at the apical surface of cortical ventricular zone compared with wild type. Het CKO mice also failed to maintain progenitor pools essential for neurogenesis. The proportion of cortical cells undergoing apoptotic cell death increased, suggesting that cell death reduces progenitor population and neuron number. Het CKO mice also showed behavioral changes, similar to our patients. To our knowledge, this is the first report to show that variants in MPP5 are associated with GDD, behavioral abnormalities and language regression/delay. Murine modeling shows that neurogenesis is likely altered in these individuals, with cell death and skewed cellular composition playing significant roles.

Functional analysis of CASK transcript variants expressed in human brain

The calcium-/calmodulin dependent serine protein kinase (CASK) belongs to the membrane-associated guanylate kinases (MAGUK) family of proteins. It fulfils several different cellular functions, ranging from acting as a scaffold protein to transcription control, as well as regulation of receptor sorting. CASK functions depend on the interaction with a variety of partners, for example neurexin, liprin-α, Tbr1 and SAP97. So far, it is uncertain how these seemingly unrelated interactions and resulting functions of CASK are regulated. Here, we show that alternative splicing of CASK can guide the binding affinity of CASK isoforms to distinct interaction partners. We report seven different variants of CASK expressed in the fetal human brain. Four out of these variants are not present in the NCBI GenBank database as known human variants. Functional analyses showed that alternative splicing affected the affinities of CASK variants for several of the tested interaction partners. Thus, we observed a clear correlation of the presence of one splice insert with poor binding of CASK to SAP97, supported by molecular modelling. The alternative splicing and distinct properties of CASK variants in terms of protein-protein interaction should be taken into consideration for future studies.

Structural basis of coronavirus E protein interactions with human PALS1 PDZ domain

SARS-CoV-2 infection leads to coronavirus disease 2019 (COVID-19), which is associated with severe and life-threatening pneumonia and respiratory failure. However, the molecular basis of these symptoms remains unclear. SARS-CoV-1 E protein interferes with control of cell polarity and cell-cell junction integrity in human epithelial cells by binding to the PALS1 PDZ domain, a key component of the Crumbs polarity complex. We show that C-terminal PDZ binding motifs of SARS-CoV-1 and SARS-CoV-2 E proteins bind the PALS1 PDZ domain with 29.6 and 22.8 μM affinity, whereas the related sequence from MERS-CoV did not bind. We then determined crystal structures of PALS1 PDZ domain bound to both SARS-CoV-1 and SARS-CoV-2 E protein PDZ binding motifs. Our findings establish the structural basis for SARS-CoV-1/2 mediated subversion of Crumbs polarity signalling and serve as a platform for the development of small molecule inhibitors to suppress SARS-CoV-1/2 mediated disruption of polarity signalling in epithelial cells.

Airah Javorsky et al. present the crystal structures of SARS-CoV-1 and SARS-CoV-2 E proteins in complex with the PALS1 PDZ domain. Their results suggest that the coronavirus E protein can interfere with normal PALS1 binding, potentially disrupting epithelial tissue integrity, and may provide future insight into the development of small molecule inhibitors against SARS-CoV-1/2.

Structural basis for SARS-CoV-2 envelope protein recognition of human cell junction protein PALS1

The COVID-19 pandemic, caused by the SARS-CoV-2 virus, has created global health and economic emergencies. SARS-CoV-2 viruses promote their own spread and virulence by hijacking human proteins, which occurs through viral protein recognition of human targets. To understand the structural basis for SARS-CoV-2 viral-host protein recognition, here we use cryo-electron microscopy (cryo-EM) to determine a complex structure of the human cell junction protein PALS1 and SARS-CoV-2 viral envelope (E) protein. Our reported structure shows that the E protein C-terminal DLLV motif recognizes a pocket formed exclusively by hydrophobic residues from the PDZ and SH3 domains of PALS1. Our structural analysis provides an explanation for the observation that the viral E protein recruits PALS1 from lung epithelial cell junctions. In addition, our structure provides novel targets for peptide- and small-molecule inhibitors that could block the PALS1-E interactions to reduce E-mediated virulence.

Crumbs2 Is an Essential Slit Diaphragm Protein of the Renal Filtration Barrier

BACKGROUND

Crumbs2 is expressed at embryonic stages as well as in the retina, brain, and glomerular podocytes. Recent studies identified CRB2 mutations as a novel cause of steroid-resistant nephrotic syndrome (SRNS).

METHODS

To study the function of Crb2 at the renal filtration barrier, mice lacking Crb2 exclusively in podocytes were generated. Gene expression and histologic studies as well as transmission and scanning electron microscopy were used to analyze these Crb2^podKO knockout mice and their littermate controls. Furthermore, high-resolution expansion microscopy was used to investigate Crb2 distribution in murine glomeruli. For pull-down experiments, live cell imaging, and transcriptome analyses, cell lines were applied that inducibly express fluorescent protein-tagged CRB2 wild type and mutants.

RESULTS

Crb2^podKO mice developed proteinuria directly after birth that preceded a prominent development of disordered and effaced foot processes, upregulation of renal injury and inflammatory markers, and glomerulosclerosis. Pull-down assays revealed an interaction of CRB2 with Nephrin, mediated by their extracellular domains. Expansion microscopy showed that in mice glomeruli, Crb2 and Nephrin are organized in adjacent clusters. SRNS-associated CRB2 protein variants and a mutant that lacks a putative conserved O-glycosylation site were not transported to the cell surface. Instead, mutants accumulated in the ER, showed altered glycosylation pattern, and triggered an ER stress response.

CONCLUSIONS

Crb2 is an essential component of the podocyte's slit diaphragm, interacting with Nephrin. Loss of slit diaphragm targeting and increasing ER stress are pivotal factors for onset and progression of CRB2-related SRNS.

Structural determinants driving the binding process between PDZ domain of wild type human PALS1 protein and SLiM sequences of SARS-CoV E proteins

Short Linear Motifs (SLiMs) are functional protein microdomains that typically mediate interactions between a short linear region in one protein and a globular domain in another. Surface Plasmon Resonance assays have been performed to determine the binding affinity between PDZ domain of wild type human PALS1 protein and tetradecapeptides representing the SLiMs sequences of SARS-CoV-1 and SARS-CoV-2 E proteins (E-SLiMs). SARS-CoV-2 E-SLiM binds to the human target protein with a higher affinity compared to SARS-CoV-1, showing a difference significantly greater than previously reported using the F318W mutant of PALS1 protein and shorter target peptides. Moreover, molecular dynamics simulations have provided clear evidence of the structural determinants driving this binding process. Specifically, the Arginine 69 residue in the SARS-CoV-2 E-SLiM is the key residue able to both enhance the specific polar interaction with negatively charged pockets of the PALS1 PDZ domain and reduce significantly the mobility of the viral peptide. These experimental and computational data are reinforced by the comparison of the interaction between the PALS1 PDZ domain with the natural ligand CRB1, as well as the corresponding E-SLiMs of other coronavirus members such as MERS and OCF43. Our results provide a model at the molecular level of the strategies used to mimic the endogenous SLiM peptide in the binding of the tight junctions of the host cell, explaining one of the possible reasons of the severity of the infection and pulmonary inflammation by SARS-CoV-2.

Recruitment of Polarity Complexes and Tight Junction Proteins to the Site of Apical Bulk Endocytosis

BACKGROUND & AIMS

The molecular motor, Myosin Vb (MYO5B), is well documented for its role in trafficking cargo to the apical membrane of epithelial cells. Despite its involvement in regulating apical proteins, the role of MYO5B in cell polarity is less clear. Inactivating mutations in MYO5B result in microvillus inclusion disease (MVID), a disorder characterized by loss of key apical transporters and the presence of intracellular inclusions in enterocytes. We previously identified that inclusions in Myo5b knockout (KO) mice form from invagination of the apical brush border via apical bulk endocytosis. Herein, we sought to elucidate the role of polarity complexes and tight junction proteins during the formation of inclusions.

METHODS

Intestinal tissue from neonatal control and Myo5b KO littermates was analyzed by immunofluorescence to determine the localization of polarity complexes and tight junction proteins.

RESULTS

Proteins that make up the apical polarity complexes-Crumbs3 and Pars complexes-were associated with inclusions in Myo5b KO mice. In addition, tight junction proteins were observed to be concentrated over inclusions that were present at the apical membrane of Myo5b-deficient enterocytes in vivo and in vitro. Our mouse findings are complemented by immunostaining in a large animal swine model of MVID genetically engineered to express a human MVID-associated mutation that shows an accumulation of Claudin-2 over forming inclusions. The findings from our swine model of MVID suggest that a similar mechanism of tight junction accumulation occurs in patients with MVID.

CONCLUSIONS

These data show that apical bulk endocytosis involves the altered localization of apical polarity proteins and tight junction proteins after loss of Myo5b.

Divergent Evolution of a Protein-Protein Interaction Revealed through Ancestral Sequence Reconstruction and Resurrection

The postsynaptic density extends across the postsynaptic dendritic spine with discs large (DLG) as the most abundant scaffolding protein. DLG dynamically alters the structure of the postsynaptic density, thus controlling the function and distribution of specific receptors at the synapse. DLG contains three PDZ domains and one important interaction governing postsynaptic architecture is that between the PDZ3 domain from DLG and a protein called cysteine-rich interactor of PDZ3 (CRIPT). However, little is known regarding functional evolution of the PDZ3:CRIPT interaction. Here, we subjected PDZ3 and CRIPT to ancestral sequence reconstruction, resurrection, and biophysical experiments. We show that the PDZ3:CRIPT interaction is an ancient interaction, which was likely present in the last common ancestor of Eukaryotes, and that high affinity is maintained in most extant animal phyla. However, affinity is low in nematodes and insects, raising questions about the physiological function of the interaction in species from these animal groups. Our findings demonstrate how an apparently established protein-protein interaction involved in cellular scaffolding in bilaterians can suddenly be subject to dynamic evolution including possible loss of function.

Missense mutations in CASK, coding for the calcium-/calmodulin-dependent serine protein kinase, interfere with neurexin binding and neurexin-induced oligomerization

Mutations in the X-linked gene coding for the calcium-/calmodulin-dependent serine protein kinase (CASK) are associated with severe neurological disorders ranging from intellectual disability (in males) to mental retardation and microcephaly with pontine and cerebellar hypoplasia. CASK is involved in transcription control, in the regulation of trafficking of the post-synaptic NMDA and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, and acts as a presynaptic scaffolding protein. For CASK missense mutations, it is mostly unclear which of CASK's molecular interactions and cellular functions are altered and contribute to patient phenotypes. We identified five CASK missense mutations in male patients affected by neurodevelopmental disorders. These and five previously reported mutations were systematically analysed with respect to interaction with CASK interaction partners by co-expression and co-immunoprecipitation. We show that one mutation in the L27 domain interferes with binding to synapse-associated protein of 97 kDa. Two mutations in the guanylate kinase (GK) domain affect binding of CASK to the nuclear factors CASK-interacting nucleosome assembly protein (CINAP) and T-box, brain, 1 (Tbr1). A total of five mutations in GK as well as PSD-95/discs large/ZO-1 (PDZ) domains affect binding of CASK to the pre-synaptic cell adhesion molecule Neurexin. Upon expression in neurons, we observe that binding to Neurexin is not required for pre-synaptic localization of CASK. We show by bimolecular fluorescence complementation assay that Neurexin induces oligomerization of CASK, and that mutations in GK and PDZ domains interfere with the Neurexin-induced oligomerization of CASK. Our data are supported by molecular modelling, where we observe that the cooperative activity of PDZ, SH3 and GK domains is required for Neurexin binding and oligomerization of CASK.

Not Just Another Scaffolding Protein Family: The Multifaceted MPPs

Membrane palmitoylated proteins (MPPs) are a subfamily of a larger group of multidomain proteins, namely, membrane-associated guanylate kinases (MAGUKs). The ubiquitous expression and multidomain structure of MPPs provide the ability to form diverse protein complexes at the cell membranes, which are involved in a wide range of cellular processes, including establishing the proper cell structure, polarity and cell adhesion. The formation of MPP-dependent complexes in various cell types seems to be based on similar principles, but involves members of different protein groups, such as 4.1-ezrin-radixin-moesin (FERM) domain-containing proteins, polarity proteins or other MAGUKs, showing their multifaceted nature. In this review, we discuss the function of the MPP family in the formation of multiple protein complexes. Notably, we depict their significant role for cell physiology, as the loss of interactions between proteins involved in the complex has a variety of negative consequences. Moreover, based on recent studies concerning the mechanism of membrane raft formation, we shed new light on a possible role played by MPPs in lateral membrane organization.

Improved binding of SARS-CoV-2 Envelope protein to tight junction-associated PALS1 could play a key role in COVID-19 pathogenesis

The Envelope (E) protein of SARS-CoV-2 is the most enigmatic protein among the four structural ones. Most of its current knowledge is based on the direct comparison to the SARS E protein, initially mistakenly undervalued and subsequently proved to be a key factor in the ER-Golgi localization and in tight junction disruption. We compared the genomic sequences of E protein of SARS-CoV-2, SARS-CoV and the closely related genomes of bats and pangolins obtained from the GISAID and GenBank databases. When compared to the known SARS E protein, we observed a significant difference in amino acid sequence in the C-terminal end of SARS-CoV-2 E protein. Subsequently, in silico modelling analyses of E proteins conformation and docking provide evidences of a strengthened binding of SARS-CoV-2 E protein with the tight junction-associated PALS1 protein. Based on our computational evidences and on data related to SARS-CoV, we believe that SARS-CoV-2 E protein interferes more stably with PALS1 leading to an enhanced epithelial barrier disruption, amplifying the inflammatory processes, and promoting tissue remodelling. These findings raise a warning on the underestimated role of the E protein in the pathogenic mechanism and open the route to detailed experimental investigations.

Multiple functions of the scaffold protein Discs large 5 in the control of growth, cell polarity and cell adhesion in Drosophila melanogaster

Background

Scaffold proteins support a variety of key processes during animal development. Mutant mouse for the MAGUK protein Discs large 5 (Dlg5) presents a general growth impairment and moderate morphogenetic defects.

Results

Here, we generated null mutants for Drosophila Dlg5 and show that it owns similar functions in growth and epithelial architecture. Dlg5 is required for growth at a cell autonomous level in several tissues and at the organism level, affecting cell size and proliferation. Our results are consistent with Dlg5 modulating hippo pathway in the wing disc, including the impact on cell size, a defect that is reproduced by the loss of yorkie. However, other observations indicate that Dlg5 regulates growth by at least another way that may involve Myc protein but nor PI3K neither TOR pathways. Moreover, epithelia cells mutant for Dlg5 also show a reduction of apical domain determinants, though not sufficient to induce a complete loss of cell polarity. Dlg5 is also essential, in the same cells, for the presence at Adherens junctions of N-Cadherin, but not E-Cadherin. Genetic analyses indicate that junction and polarity defects are independent.

Conclusions

Together our data show that Dlg5 own several conserved functions that are independent of each other in regulating growth, cell polarity and cell adhesion. Moreover, they reveal a differential regulation of E-cadherin and N-cadherin apical localization.

The cytoskeletal motor proteins Dynein and MyoV direct apical transport of Crumbs

Crumbs (Crb in Drosophila; CRB1-3 in mammals) is a transmembrane determinant of epithelial cell polarity and a regulator of Hippo signalling. Crb is normally localized to apical cell-cell contacts, just above adherens junctions, but how apical trafficking of Crb is regulated in epithelial cells remains unclear. We use the Drosophila follicular epithelium to demonstrate that polarized trafficking of Crb is mediated by transport along microtubules by the motor protein Dynein and along actin filaments by the motor protein Myosin-V (MyoV). Blocking transport of Crb-containing vesicles by Dynein or MyoV leads to accumulation of Crb within Rab11 endosomes, rather than apical delivery. The final steps of Crb delivery and stabilisation at the plasma membrane requires the exocyst complex and three apical FERM domain proteins - Merlin, Moesin and Expanded - whose simultaneous loss disrupts apical localization of Crb. Accordingly, a knock-in deletion of the Crb FERM-binding motif (FBM) also impairs apical localization. Finally, overexpression of Crb challenges this system, creating a sensitized background to identify components involved in cytoskeletal polarization, apical membrane trafficking and stabilisation of Crb at the apical domain.

Structural Features of Tight-Junction Proteins

Tight junctions are complex supramolecular entities composed of integral membrane proteins, membrane-associated and soluble cytoplasmic proteins engaging in an intricate and dynamic system of protein-protein interactions. Three-dimensional structures of several tight-junction proteins or their isolated domains have been determined by X-ray crystallography, nuclear magnetic resonance spectroscopy, and cryo-electron microscopy. These structures provide direct insight into molecular interactions that contribute to the formation, integrity, or function of tight junctions. In addition, the known experimental structures have allowed the modeling of ligand-binding events involving tight-junction proteins. Here, we review the published structures of tight-junction proteins. We show that these proteins are composed of a limited set of structural motifs and highlight common types of interactions between tight-junction proteins and their ligands involving these motifs.

Crumbs organizes the transport machinery by regulating apical levels of PI(4,5)P2 in Drosophila

An efficient vectorial intracellular transport machinery depends on a well-established apico-basal polarity and is a prerequisite for the function of secretory epithelia. Despite extensive knowledge on individual trafficking pathways, little is known about the mechanisms coordinating their temporal and spatial regulation. Here, we report that the polarity protein Crumbs is essential for apical plasma membrane phospholipid-homeostasis and efficient apical secretion. Through recruiting βHeavy-Spectrin and MyosinV to the apical membrane, Crumbs maintains the Rab6-, Rab11- and Rab30-dependent trafficking and regulates the lipid phosphatases Pten and Ocrl. Crumbs knock-down results in increased apical levels of PI(4,5)P2 and formation of a novel, Moesin- and PI(4,5)P2-enriched apical membrane sac containing microvilli-like structures. Our results identify Crumbs as an essential hub required to maintain the organization of the apical membrane and the physiological activity of the larval salivary gland.

The WW1 Domain Enhances Autoinhibition in Smurf Ubiquitin Ligases

Downregulation of ubiquitin (Ub) ligase activity prevents premature ubiquitination and is critical for cellular homeostasis. Nedd4 Ub ligases share a common domain architecture and yet are regulated in distinct ways through interactions of the catalytic HECT domain with the N-terminal C2 domain or the central WW domain region. Smurf1 and Smurf2 are two highly related Nedd4 ligases with ~70% overall sequence identity. Here, we show that the Smurf1 C2 domain interacts with the HECT domain and inhibits ligase activity in trans. However, in contrast to Smurf2, we find that full-length Smurf1 is a highly active Ub ligase, and we can attribute this striking difference in regulation to the lack of one WW domain (WW1) in Smurf1. Using NMR spectroscopy and biochemical assays, we identified the WW1 region as an additional inhibitory element in Smurf2 that cooperates with the C2 domain to enhance HECT domain binding and Smurf2 inhibition. Our work provides important insights into Smurf regulation and highlights that the activities of highly related proteins can be controlled in distinct ways.

PDZ Domains as Drug Targets

Protein-protein interactions within protein networks shape the human interactome, which often is promoted by specialized protein interaction modules, such as the postsynaptic density-95 (PSD-95), discs-large, zona occludens 1 (ZO-1) (PDZ) domains. PDZ domains play a role in several cellular functions, from cell-cell communication and polarization, to regulation of protein transport and protein metabolism. PDZ domain proteins are also crucial in the formation and stability of protein complexes, establishing an important bridge between extracellular stimuli detected by transmembrane receptors and intracellular responses. PDZ domains have been suggested as promising drug targets in several diseases, ranging from neurological and oncological disorders to viral infections. In this review, the authors describe structural and genetic aspects of PDZ-containing proteins and discuss the current status of the development of small-molecule and peptide modulators of PDZ domains. An overview of potential new therapeutic interventions in PDZ-mediated protein networks is also provided.

The Crumbs3 Polarity Protein

The Crumbs proteins are evolutionarily conserved apical transmembrane proteins. Drosophila Crumbs was discovered via its crucial role in epithelial polarity during fly embryogenesis. Crumbs proteins have variable extracellular domains but a highly conserved intracellular domain that can bind FERM and PDZ domain proteins. Mammals have three Crumbs genes and this review focuses on Crumbs3, the major Crumbs isoform expressed in mammalian epithelial cells. Although initial work has highlighted the role of Crumbs3 in polarity, more recent studies have found it has an important role in tissue morphogenesis functioning as a linker between the apical membrane and the actin cytoskeleton. In addition, recent publications have linked Crumbs3 to growth control via regulation of the Hippo/Yap pathway.

Two microcephaly-associated novel missense mutations in CASK specifically disrupt the CASK-neurexin interaction

Deletion and truncation mutations in the X-linked gene CASK are associated with severe intellectual disability (ID), microcephaly and pontine and cerebellar hypoplasia in girls (MICPCH). The molecular origin of CASK-linked MICPCH is presumed to be due to disruption of the CASK-Tbr-1 interaction. This hypothesis, however, has not been directly tested. Missense variants in CASK are typically asymptomatic in girls. We report three severely affected girls with heterozygous CASK missense mutations (M519T (2), G659D (1)) who exhibit ID, microcephaly, and hindbrain hypoplasia. The mutation M519T results in the replacement of an evolutionarily invariant methionine located in the PDZ signaling domain known to be critical for the CASK-neurexin interaction. CASK^M519T is incapable of binding to neurexin, suggesting a critically important role for the CASK-neurexin interaction. The mutation G659D is in the SH3 (Src homology 3) domain of CASK, replacing a semi-conserved glycine with aspartate. We demonstrate that the CASK^G659D mutation affects the CASK protein in two independent ways: (1) it increases the protein's propensity to aggregate; and (2) it disrupts the interface between CASK's PDZ (PSD95, Dlg, ZO-1) and SH3 domains, inhibiting the CASK-neurexin interaction despite residing outside of the domain deemed critical for neurexin interaction. Since heterozygosity of other aggregation-inducing mutations (e.g., CASK^W919R) does not produce MICPCH, we suggest that the G659D mutation produces microcephaly by disrupting the CASK-neurexin interaction. Our results suggest that disruption of the CASK-neurexin interaction, not the CASK-Tbr-1 interaction, produces microcephaly and cerebellar hypoplasia. These findings underscore the importance of functional validation for variant classification.

PDZ Ligand Binding-Induced Conformational Coupling of the PDZ-SH3-GK Tandems in PSD-95 Family MAGUKs

Discs large (DLG) MAGUKs are abundantly expressed in glutamatergic synapses, crucial for synaptic transmission, and plasticity by anchoring various postsynaptic components including glutamate receptors, downstream scaffold proteins and signaling enzymes. Different DLG members have shared structures and functions, but also contain unique features. How DLG family proteins function individually and cooperatively is largely unknown. Here, we report that PSD-95 PDZ3 directly couples with SH3-GK tandem in a PDZ ligand binding-dependent manner, and the coupling can promote PSD-95 dimerization and multimerization. Aided by sortase-mediated protein ligation and selectively labeling, we elucidated the PDZ3/SH3-GK conformational coupling mechanism using NMR spectroscopy. We further demonstrated that PSD-93, but not SAP102, can also undergo PDZ3 ligand binding-induced conformational coupling with SH3-GK and form homo-oligomers. Interestingly, PSD-95 and PSD-93 can also form ligand binding-induced hetero-oligomers, suggesting a cooperative assembly mechanism for the mega-N-methyl-d-aspartate receptor synaptic signaling complex. Finally, we provide evidence showing that ligand binding-induced conformational coupling between PDZ and SH3-GK is a common feature for other MAGUKs including CASK and PALS1.

Optic Nerve Hypoplasia Is a Pervasive Subcortical Pathology of Visual System in Neonates

Purpose

Optic nerve hypoplasia (ONH) is the most common cause of childhood congenital blindness in developed nations, yet the fundamental pathobiology of ONH remains unknown. The objective of this study was to employ a ‘face validated' murine model to determine the timing of onset and the pathologic characteristics of ONH.

Methods

Based on the robust linkage between X-linked CASK haploinsufficiency and clinically diagnosed ONH, we hypothesized that heterozygous deletion of CASK (CASK^(+/−)) in rodents will produce an optic nerve pathology closely recapitulating ONH. We quantitatively analyzed the entire subcortical visual system in female CASK^(+/−) mice using immunohistochemistry, anterograde axonal tracing, toluidine blue staining, transmission electron microscopy, and serial block-face scanning electron microscopy.

Results

CASK haploinsuffiency in mice phenocopies human ONH with complete penetrance, thus satisfying the ‘face validity'. We demonstrate that the optic nerve in CASK^(+/−) mice is not only thin, but is comprised of atrophic retinal axons and displays reactive astrogliosis. Myelination of the optic nerve axons remains unchanged. Moreover, we demonstrate a significant decrease in retinal ganglion cell (RGC) numbers and perturbation in retinothalamic connectivity. Finally, we used this mouse model to define the onset and progression of ONH pathology, demonstrating for the first time that optic nerve defects arise at neonatally in CASK^(+/−)mice.

Conclusions

Optic nerve hypoplasia is a complex neuropathology of the subcortical visual system involving RGC loss, axonopathy, and synaptopathy and originates at a developmental stage in mice that corresponds to the late third trimester development in humans.

Protein Complex Assemblies in Epithelial Cell Polarity and Asymmetric Cell Division

Asymmetric local concentration of protein complexes on distinct membrane regions is a fundamental property in numerous biological processes and is a hallmark of cell polarity. Evolutionarily conserved core polarity proteins form specific and dynamic networks to regulate the establishment and maintenance of cell polarity, as well as distinct polarity-driven cellular events. This review focuses on the molecular and structural basis governing regulated formation of several sets of core cell polarity regulatory complexes, as well as their functions in epithelial cell polarization and asymmetric cell division.

Mechanisms of MAGUK-mediated cellular junctional complex organization

Membrane-associated guanylate kinases (MAGUKs) are a family of scaffold proteins that are enriched in cellular junctions and essential for tissue development and homeostasis. Mutations of MAGUKs are linked to many human diseases including cancers, psychiatric disorders, and intellectual disabilities. MAGUKs share a common PDZ-SH3-GK tandem domain organization at the C-terminal end. In this review, we summarize the mechanistic basis governing target recognition and regulations of this binding by the PDZ-SH3-GK tandem of various MAGUKs. We also discuss recent discoveries showing unique folding features of MAGUK PDZ-SH3-GK tandems that facilitate ligand-induced oligomerization of MAGUKs and phase transition of MAGUK-assembled synaptic signaling complexes.

Supramolecular organization of NMDA receptors and the postsynaptic density

The postsynaptic density (PSD) of all vertebrate species share a highly complex proteome with ∼1000 conserved proteins that function as sophisticated molecular computational devices. Here, we review recent studies showing that this complexity can be understood in terms of the supramolecular organization of proteins, which self-assemble within a hierarchy of different length scales, including complexes, supercomplexes and nanodomains. We highlight how genetic and biochemical approaches in mice are being used to uncover the native molecular architecture of the synapse, revealing hitherto unknown molecular structures, including highly selective mechanisms for specifying the assembly of NMDAR-MAGUK supercomplexes. We propose there exists a logical framework that precisely dictates the subunit composition of synaptic complexes, supercomplexes, and nanodomains in vivo.

EPEC effector EspF promotes Crumbs3 endocytosis and disrupts epithelial cell polarity

Enteropathogenic Escherichia coli (EPEC) uses a type III secretion system to inject effector proteins into host intestinal epithelial cells causing diarrhoea. EPEC infection redistributes basolateral proteins β1-integrin and Na⁺ /K⁺ ATPase to the apical membrane of host cells. The Crumbs (Crb) polarity complex (Crb3/Pals1/Patj) is essential for epithelial cell polarisation and tight junction (TJ) assembly. Here, we demonstrate that EPEC displaces Crb3 and Pals1 from the apical membrane to the cytoplasm of cultured intestinal epithelial cells and colonocytes of infected mice. In vitro studies show that EspF, but not Map, alters Crb3, whereas both effectors modulate Pals1. EspF perturbs polarity formation in cyst morphogenesis assays and induces endocytosis and apical redistribution of Na⁺ /K⁺ ATPase. EspF binds to sorting nexin 9 (SNX9) causing membrane remodelling in host cells. Infection with ΔespF/pespFD3, a mutant strain that ablates EspF binding to SNX9, or inhibition of dynamin, attenuates Crb3 endocytosis caused by EPEC. In addition, infection with ΔespF/pespFD3 has no impact on Na⁺ /K⁺ ATPase endocytosis. These data support the hypothesis that EPEC perturbs apical-basal polarity in an EspF-dependent manner, which would contribute to EPEC-associated diarrhoea by disruption of TJ and altering the crucial positioning of membrane transporters involved in the absorption of ions and solutes.

Neuralized regulates Crumbs endocytosis and epithelium morphogenesis via specific Stardust isoforms

The E3 ubiquitin ligase Neuralized is shown to interact with a subset of the Stardust isoforms to regulate the endocytosis of the apical protein Crumbs and thereby promote epithelial remodeling during Drosophila development.

Crumbs (Crb) is a conserved determinant of apical membrane identity that regulates epithelial morphogenesis in many developmental contexts. In this study, we identify the Crb complex protein Stardust (Sdt) as a target of the E3 ubiquitin ligase Neuralized (Neur) in Drosophila melanogaster. Neur interacts with and down-regulates specific Sdt isoforms containing a Neur binding motif (NBM). Using a CRISPR (clustered regularly interspaced short palindromic repeats)-induced deletion of the NBM-encoding exon, we found that Sdt is a key Neur target and that Neur acts via Sdt to down-regulate Crb. We further show that Neur promotes the endocytosis of Crb via the NBM-containing isoforms of Sdt. Although the regulation of Crb by Neur is not strictly essential, it contributes to epithelium remodeling in the posterior midgut and thereby facilitates the trans-epithelial migration of the primordial germ cells in early embryos. Thus, our study uncovers a novel regulatory mechanism for the developmental control of Crb-mediated morphogenesis.

The CRB1 Complex: Following the Trail of Crumbs to a Feasible Gene Therapy Strategy

Once considered science fiction, gene therapy is rapidly becoming scientific reality, targeting a growing number of the approximately 250 genes linked to hereditary retinal disorders such as retinitis pigmentosa and Leber's congenital amaurosis. Powerful new technologies have emerged, leading to the development of humanized models for testing and screening these therapies, bringing us closer to the goal of personalized medicine. These tools include the ability to differentiate human induced pluripotent stem cells (iPSCs) to create a “retina-in-a-dish” model and the self-formed ectodermal autonomous multi-zone, which can mimic whole eye development. In addition, highly specific gene-editing tools are now available, including the CRISPR/Cas9 system and the recently developed homology-independent targeted integration approach, which allows gene editing in non-dividing cells. Variants in the CRB1 gene have long been associated with retinopathies, and more recently the CRB2 gene has also been shown to have possible clinical relevance with respect to retinopathies. In this review, we discuss the role of the CRB protein complex in patients with retinopathy. In addition, we discuss new opportunities provided by stem cells and gene-editing tools, and we provide insight into how the retinal therapeutic pipeline can be improved. Finally, we discuss the current state of adeno-associated virus-mediated gene therapy and how it can be applied to treat retinopathies associated with mutations in CRB1.

Phase Transition in Postsynaptic Densities Underlies Formation of Synaptic Complexes and Synaptic Plasticity

Postsynaptic densities (PSDs) are membrane semi-enclosed, submicron protein-enriched cellular compartments beneath postsynaptic membranes, which constantly exchange their components with bulk aqueous cytoplasm in synaptic spines. Formation and activity-dependent modulation of PSDs is considered as one of the most basic molecular events governing synaptic plasticity in the nervous system. In this study, we discover that SynGAP, one of the most abundant PSD proteins and a Ras/Rap GTPase activator, forms a homo-trimer and binds to multiple copies of PSD-95. Binding of SynGAP to PSD-95 induces phase separation of the complex, forming highly concentrated liquid-like droplets reminiscent of the PSD. The multivalent nature of the SynGAP/PSD-95 complex is critical for the phase separation to occur and for proper activity-dependent SynGAP dispersions from the PSD. In addition to revealing a dynamic anchoring mechanism of SynGAP at the PSD, our results also suggest a model for phase-transition-mediated formation of PSD.

Domain-specific functions of Stardust in Drosophila embryonic development

In Drosophila, the adaptor protein Stardust is essential for the stabilization of the polarity determinant Crumbs in various epithelial tissues, including the embryonic epidermis, the follicular epithelium and photoreceptor cells of the compound eye. In turn, Stardust recruits another adaptor protein, PATJ, to the subapical region to support adherens junction formation and morphogenetic events. Moreover, Stardust binds to Lin-7, which is dispensable in epithelial cells but functions in postsynaptic vesicle fusion. Finally, Stardust has been reported to bind directly to PAR-6, thereby linking the Crumbs-Stardust-PATJ complex to the PAR-6/aPKC complex. PAR-6 and aPKC are also capable of directly binding Bazooka (the Drosophila homologue of PAR-3) to form the PAR/aPKC complex, which is essential for apical-basal polarity and cell-cell contact formation in most epithelia. However, little is known about the physiological relevance of these interactions in the embryonic epidermis of Drosophila in vivo. Thus, we performed a structure-function analysis of the annotated domains with GFP-tagged Stardust and evaluated the localization and function of the mutant proteins in epithelial cells of the embryonic epidermis. The data presented here confirm a crucial role of the PDZ domain in binding Crumbs and recruiting the protein to the subapical region. However, the isolated PDZ domain is not capable of being recruited to the cortex, and the SH3 domain is essential to support the binding to Crumbs. Notably, the conserved N-terminal regions (ECR1 and ECR2) are not crucial for epithelial polarity. Finally, the GUK domain plays an important role for the protein's function, which is not directly linked to Crumbs stabilization, and the L27N domain is essential for epithelial polarization independently of recruiting PATJ.

MPP2 is a postsynaptic MAGUK scaffold protein that links SynCAM1 cell adhesion molecules to core components of the postsynaptic density

At neuronal synapses, multiprotein complexes of trans-synaptic adhesion molecules, scaffold proteins and neurotransmitter receptors assemble to essential building blocks required for synapse formation and maintenance. Here we describe a novel role for the membrane-associated guanylate kinase (MAGUK) protein MPP2 (MAGUK p55 subfamily member 2) at synapses of rat central neurons. Through interactions mediated by its C-terminal SH3-GK domain module, MPP2 binds to the abundant postsynaptic scaffold proteins PSD-95 and GKAP and localises to postsynaptic sites in hippocampal neurons. MPP2 also colocalises with the synaptic adhesion molecule SynCAM1. We demonstrate that the SynCAM1 C-terminus interacts directly with the MPP2 PDZ domain and that MPP2 does not interact in this manner with other highly abundant postsynaptic transmembrane proteins. Our results highlight a previously unexplored role for MPP2 at postsynaptic sites as a scaffold that links SynCAM1 cell adhesion molecules to core proteins of the postsynaptic density.

Crumbs 3b promotes tight junctions in an ezrin-dependent manner in mammalian cells

Crumbs 3 (CRB3) is a component of epithelial junctions, which has been implicated in apical-basal polarity, apical identity, apical stability, cell adhesion, and cell growth. CRB3 undergoes alternative splicing to yield two variants: CRB3a and CRB3b. Here, we describe novel data demonstrating that, as with previous studies on CRB3a, CRB3b also promotes the formation of tight junctions (TJs). However, significantly we demonstrate that the 4.1-ezrin-radixin-moesin-binding motif of CRB3b is required for CRB3b functionality and that ezrin binds to the FBM of CRB3b. Furthermore, we show that ezrin contributes to CRB3b functionality and the correct distribution of TJ proteins. We demonstrate that both CRB3 isoforms are required for the production of functionally mature TJs and also the localization of ezrin to the plasma membrane. Finally, we demonstrate that reduced CRB3b expression in head and neck squamous cell carcinoma (HNSCC) correlates with cytoplasmic ezrin, a biomarker for aggressive disease, and shows evidence that while CRB3a expression has no effect, low CRB3b and high cytoplasmic ezrin expression combined may be prognostic for HNSCC.

An Atypical MAGUK GK Target Recognition Mode Revealed by the Interaction between DLG and KIF13B

The membrane-associated guanylate kinase (MAGUK) scaffold proteins share a signature guanylate kinase (GK) domain. Despite their diverse functional roles in cell polarity control and synaptic signaling, the currently known mode of action of MAGUK GK is via its binding to phosphorylated short peptides from target proteins. Here, we discover that the GK domain of DLG MAGUK binds to an unphosphorylated and autonomously folded domain within the stalk region (MAGUK binding stalk [MBS] domain) of a kinesin motor KIF13B with high specificity and affinity. The structure of DLG4 GK in complex with KIF13B MBS reveals the molecular mechanism governing this atypical GK/target recognition mode and provides insights into DLG/KIF13B complex-mediated regulation of diverse cellular processes such as asymmetric cell division. We further show that binding to non-phosphorylated targets is another general property of MAGUK GKs, thus expanding the mechanisms of action of the MAGUK family proteins.

Cell polarity proteins and spermatogenesis

When the cross-section of a seminiferous tubule from an adult rat testes is examined microscopically, Sertoli cells and germ cells in the seminiferous epithelium are notably polarized cells. For instance, Sertoli cell nuclei are found near the basement membrane. On the other hand, tight junction (TJ), basal ectoplasmic specialization (basal ES, a testis-specific actin-rich anchoring junction), gap junction (GJ) and desmosome that constitute the blood-testis barrier (BTB) are also located near the basement membrane. The BTB, in turn, divides the epithelium into the basal and the adluminal (apical) compartments. Within the epithelium, undifferentiated spermatogonia and preleptotene spermatocytes restrictively reside in the basal compartment whereas spermatocytes and post-meiotic spermatids reside in the adluminal compartment. Furthermore, the heads of elongating/elongated spermatids point toward the basement membrane with their elongating tails toward the tubule lumen. However, the involvement of polarity proteins in this unique cellular organization, in particular the underlying molecular mechanism(s) by which polarity proteins confer cellular polarity in the seminiferous epithelium is virtually unknown until recent years. Herein, we discuss latest findings regarding the role of different polarity protein complexes or modules and how these protein complexes are working in concert to modulate Sertoli cell and spermatid polarity. These findings also illustrate polarity proteins exert their effects through the actin-based cytoskeleton mediated by actin binding and regulatory proteins, which in turn modulate adhesion protein complexes at the cell-cell interface since TJ, basal ES and GJ utilize F-actin for attachment. We also propose a hypothetical model which illustrates the antagonistic effects of these polarity proteins. This in turn provides a unique mechanism to modulate junction remodeling in the testis to support germ cell transport across the epithelium in particular the BTB during the epithelial cycle of spermatogenesis.